1. Field of the Invention
This invention relates generally to reading and writing data on a magnetic storage medium and in particular to reading and writing data at a predetermined location on a magnetic storage medium when the center of the magnetic storage medium is displaced from the true center of rotation.
2. Prior Art
Typically, a disk drive contains one or more circular planar disks that are coated on each side with a magnetic medium. The disk or disks are mounted on a spindle that extends through the center of each disk so that the disks may be rotated at a predetermined speed, usually about 3600 rpm. Usually, one read/write head is associated with each side of the disk that is coated with a magnetic medium. The read/write head flies a small distance above the disk surface as the disk rotates. The read/write head, in response to signals from electronics associated with the disk drive, writes data at a predetermined location in the magnetic medium. Similarly, the read/write head, in response to other signals from the disk drive electronics, reads the stored data at a predetermined location.
The configuration of the data on the magnetic surface is instrumental in the operation of the disk drive. Data are recorded by read/write head 120 in concentric circular tracks 110 on disk 100 (FIG. 1). Corresponding tracks on different disk surfaces are cylindrically aligned.
Typically, each track is segmented into one or more parts that are referred to as sectors, e.g., sector 106, sector 107. Thus, the disk drive must move read/write head 120 radially across the disk surface to locate the track for reading or writing data and then must follow that track circumferentially until the desired sector passes under read/write head 120. Hence, read/write head 120 is positioned at a predetermined radial and circumferential position over the disk surface.
In a disk drive, each read/write head is usually affixed by an arm to a actuator and the actuator is moved so that the read/write head is moved radially to a specified track. This operation is referred to as a track seek, or sometimes just a seek. In a closed-loop disk drive, a servo system is used to move the actuator.
Many different servo systems have been developed for use in hard disk drives. In an embedded servo system, the read/write head reads a servo pattern contained in a servo field at the start of each sector to determine the radial and circumferential position of the read/write head relative to the disk. The information that is read is provided to the disk drive control loop electronics which in turn repositions the read/write head as necessary based on that information.
In large hard disk drives i.e., disk drives with a form factor of 3.5 inches or larger, the disks were clamped to the spindle with sufficient pressure to prevent displacement of the disk relative to the spindle by either starting torque or operational shock and vibration. To prevent distortion of the disks by the high clamping forces, the disk substrate was relatively stiff and as thick as 0.150 inches.
Unfortunately, the large hard disk drives are not suitable for use in portable computers because of the high profile and large form factor. A disk drive used in a portable computer preferably has a low profile and a 2.5 inch or smaller form factor. Such a disk drive is referred to herein as a miniature disk drive. The low profile design requires the use of thinner disk substrates which in some cases are as thin as 0.025 inches. Further, the low profile requires a compatible disk clamp. The reduction in the size of the disk clamp and the disk reduces the clamping region of the disk.
If the traditional high clamping forces are applied to the smaller clamping region, warpage and distortion of the disk substrate is highly probable. Such distortions are likely to result in data errors and even head crashes. Consequently, low profile disk clamps typically do not generate sufficient pressure on the disk to prevent radial slippage of the disk relative to the spindle. This problems is exacerbated because the shock and vibration that a miniature drive is subjected to in a portable computer may be up to 10 times greater than the shock and vibration in the large disk drives described above.
Thus, in miniature disk drives in general and in particular in small miniature disk drives in portable computers, shock and vibration forces on the disk drive will result in a random radial displacement between the disk and the spindle. The center of the spindle is referred to as the true center of rotation of the disk drive. As explained above, data are typically stored in concentric tracks on the disk. However, the random radial displacement of the disk would result in data tracks that are no longer concentric.
For example, initially disk 100 (FIG. 1) is centered about true center of rotation 101. Track 110 is written while disk 100 is in the centered position. A shock displaces disk 100 so that the center of the disk is located at position 102. Consequently, as the disk spins, read/write head 120 traverses over track 111. If data is now written on disk 100 in track 111, data in tracks concentric with track 110 are obliterated. Positional difference 130 between track 110 and track 111 is referred to as runout.
A practical closed loop servo system, such as that described above, can correct ninety percent of the read/write head positioning error. However, if the shock displaced disk 100 about 0.001 inches, the residual track positioning error for such a closed loop servo system is about .+-.100 microinches. If disk 100 has 2500 tracks per inch, the width of each data track is about 340 microinches with guard bands of 60 microinches. Thus, the position correction provided by the closed loop servo system is not sufficient to prevent new data from overwriting data written on adjacent tracks prior to the displacement.
If miniature disk drives are to be reliably used in portable computers, runout compensation is required beyond that provided by the closed loop servo system. One prior art runout compensation system for a large hard disk drive with a removable disk pack is described in U.S. Pat. No. 4,628,379 issued to Thomas L. Andrews, Jr. et al. on Dec. 9, 1986, which is incorporated herein by reference. In this system, servo field position information is read by read/write head 230 (FIG. 2) and provided to position determining circuit 232. Position determining circuit 232 provides information to sector index determining circuit 238 which in turn provides microprocessor 240 with index and sector numbers.
Position determining circuit 232 provides an analog position error signal to analog to digital (A/D) converter 236 and to hardware summing junction 246. A/D converter 236 sends microprocessor 240 a digital position error signal. In the runout processing performed in microprocessor 240, the runout is represented as a sinusoid using only the fundamental frequency and the offset average. A discrete Fourier transform for a single frequency is performed. To obtain sufficient data for the runout compensation, a track is sampled eight complete times to obtain an average distance value for each sector in the track.
After microprocessor 240 performs the Fourier analysis, microprocessor 240 provides digital-to-analog (D/A) converter 244 a digital compensating signal. D/A converter 244 in turn provides summing junction 248 with an analog signal. Summing junction 248 combines the position error signal from circuit 232 and the analog compensating signal from D/A converter 249 and provides the resulting signal to hardware compensator 248. Compensator 248 generates a signal that is amplified and used to reposition read/write head 230.
This compensation system was used in a relatively low density disk drive that had removable disk packs. The runout compensation was for the problems associated with re-establishing the center of rotation after reseating the disk pack on various spindles under random physical conditions.
A serious limitation of this runout compensation system is that there is not a basis for determining when the runout compensation should be applied. For the removable disk pack, the runout compensation was used for a single track on each disk surface upon power-up. The runout compensation was not suitable for use on a real-time basis during disk drive operation. For example, sampling one track on one disk surface for eight revolutions prior to a seek requires at least 133 milliseconds which is an unacceptable seek time.
Since the runout may change during operation in miniature disk drives, performing runout compensation only at power-up, as for the removable disk pack, is not acceptable. Moreover, there is no indicator that the disk has been displaced and so no basis for determining when to use the runout compensation. Thus, the system described above is not practical in a miniature disk drive. Consequently, while the need for runout compensation is recognized, a practical solution with reasonable time performance and adequate compensation on a real-time basis is needed.